Wire coating using a liquid polymer

ABSTRACT

In accordance with a preferred embodiment of this invention there is provided a method and apparatus for coating a wire or cable with a thermosetting polymer formed from a &#34;liquid polymer&#34; system such as a two-component polysiloxane. This system may be characterized as having two components each having a low viscosity and which when mixed react quickly to form the cross-linked polysiloxane rubber.

The present invention relates to extrusion or wire coating processes andequipment and, more specifically, to such processes and equipmentcapable of employing the newly developed, low viscosity multicomponentpolymer systems often termed "liquid polymers."

Most commonly used extrusion, and more particularly wire and cablecoating, processes employ thermoplastic polymer resins which, even whenmelted, are thick viscous materials. These resins are typically in solidpellet form as they are fed into the extruder or wire coater. Once inthe equipment the pellets are heated, melted and extruded in the desiredshape and then quenched to the solid state. However, at no point in theprocess is the viscosity of these materials very low. As a result,considerable energy and time are required to form these resins into thedesired shape.

Recently, several types of low viscosity or "liquid polymer"formulations have been developed. Such formations have been suggestedfor both organic and inorganic (e.g., silicone) polymers. In thisspecification, the term "liquid polymer" will be used to designate thosepolymers in which two or more reactive liquid ingredients, havingrelatively low viscosities, are blended to form a rapidly reactingmixture which cures to form a solid crosslinked polymer.

It is an object of this invention to provide a new and improved highspeed extruder specifically designed to extrude organic or inorganic"liquid polymers" and to more fully exploit their advantages. In thepractice of this invention, the low viscosity liquid components, whicheventually react to form the desired product, are preheated and pumpedinto the extruder under pressure and this initial pressure is theprimary force which mixes the components and which carries them throughthe extruder. The subject extruder is able to process the polymersseveral times faster than conventional processes and one feature thatcontributes to this increased speed is that the extruder does not dependon a conventional rotating screw to transport or mix the components.

It is an additional object of this invention to provide a new andimproved high speed wire coater specifically designed to employ organicor inorganic "liquid polymers." The low viscosity liquid components,which eventually react to form the desired polymer coating are preheatedand pumped into the subject wire coater under pressure and this initialpressure is the primary force which mixes and carries the componentsthrough the subject wire coater. In addition, the subject coating has alower incidence of pin hole defects than coatings produced by processesemploying conventional polymer resins.

It is a still further object of this invention to provide a high speedwire coater specifically adapted to rapidly coat a wire or cable with acured silicone elastomer formed from a preheated and platinum catalyzed"liquid polymer" in which the curing reaction is the Si--H additionacross a carbon-carbon double bond and which elastomer does not requirea post-coating, heat curing step.

In accordance with a preferred embodiment of this invention, these andother objects are accomplished by heating and pumping the components ofa liquid polymer into a wire coater having a die which is held in abarrel which has a tapered bore in fluid communication with the dieopening. The barrel is mounted on a supporting base member. A mandrelhaving a tapered end is positioned inside the barrel with the taperedend pointed toward the die; preferably the mandrel is rotated during thewire coating operation. The inner surface of the barrel and themandrel's surface form a mixing chamber through which the liquid polymercomponents pass, hot and under pressure, and in which they arethoroughly mixed to form a rapidly reacting mixture as they are forcedthrough the coater. To ensure an adequate degree of mixing, either themandrel's surface in the mixing chamber or the inner surface of thebarrel or both may be equipped with mixing blades or fins.

The individual "liquid polymer" components are channeled into the mixingchamber, under pressure, through toroidal distribution rings which arepositioned on the mandrel at a point so that as soon as the separatecomponents leave the rings, they enter the mixing chamber. Eachdistribution ring receives one component which is preheated and underpressure through separate inlet ports in the barrel.

It is to be noted that neither the mixing chamber nor any other elementof the coater assembly actually pumps the components or the reactingmixture toward the die. The "liquid polymer" components are introducedinto the coater under pressure and it is this initial pressure whichforces the material through the coater. The practitioner of thisinvention may control the throughput of the subject coater bycontrolling the initial pressure on the components as they enter thecoater.

This invention will provide the wire and cable industry with a processwhich will operate several times faster than the presently usedprocesses and will consume considerably less energy. In addition, it isbelieved that the use of the subject process will substantially reducethe frequency of pin hole defects since the viscosity of the subjectcoating material, as it goes on the wire, will be lower than theviscosity of the thermoplastic resins at this point. The subjectinvention will also provide a high speed process that produces a coatingwhich is stronger, more dimensionally stable and more solvent resistantthan the prior art processes. Many of these advantages are attributablein part to the nature of the coating provided by the subject process.

Applicant believes that in the past, this combination of advantages wasnot attainable since insofar as he is aware there was no commerciallyavailable wire coating process which used a "liquid polymer."

It is to be emphasized that the term "liquid polymer" is a term of artand is used to designate a thermosetting polymer formed by a rapidchemical reaction. The liquid reactive components (hereinafter"components") which react to form the polymeric product have relativelylow viscosity and are typically stored separately to ensure stabilityand a reasonable shelf life. However, inhibitors may be used to allowthe reactants to be mixed and form a storage-stable one componentprecursor which when heated rapidly reacts to form the desired thermosetpolymer.

Further objects and advantages of the present invention will becomereadily apparent to those skilled in the art from the following detaileddescription in conjunction with the accompanying drawings in which:

FIG. 1 is an elevated cut-away perspective view of the subject wirecoater;

FIG. 2 is an elevated cut-away perspective view of a distribution ringwhich uniformly feeds one component into the mixing chamber; and

FIG. 3 is a representative view of the subject wire coater and itsauxiliary equipment which includes a pump 3 and a heater 5 for eachcomponent.

Preferably, the "liquid polymer" has two low viscosity components. Bothcontain an organosilicone prepolymer having an aliphatically unsaturatedpendent group, typically a vinyl group; examples of this prepolymer aredescribed in U.S. Pat. Nos. 2,823,218; 3,419,593 and 3,697,473. Thisprepolymer is typically a polysiloxane oligomer. This first componentalso contains a platinum catalyst such as chloroplatinic acid. Thesecond component also contains the prepolymer and, in addition, acrosslinking agent which is an organosilicon compound having at leasttwo hydrogen atoms linked to a silicon atom per molecule. Thiscrosslinking agent is often referred to as an SiH containing reactantsince the SiH bond is the reactive moiety.

The two components will react very quickly, especially when heated toform a thermosetting (i.e., crosslinked) polysiloxane elastomer. Thisreaction is the SiH addition across the carbon-carbon double bond.

Preferably, the organosilicon prepolymer is a linear dimethylsiloxaneoligomer having occasional methyl groups in terminal or pendentpositions replaced by vinyl groups. The viscosity of the prepolymer isin the range of from about 20 to 4000 centipoise (1 pascal·second =10.00 poise) as measured by a Brookfield viscometer at room temperature.The SiH compound is a linear dimethylsiloxane oligomer having occasionalpendent methyl groups replaced by hydrogen. The number of hydrogengroups in the SiH compound and the number of vinyl groups in theprepolymer are adjusted so that the SiH to SiVi ratio is within therange of from 1 to 2 and preferably from about 1.3 to 1.6. The optimumSiH to SiVi ratio will depend on the particular compounds used; however,the SiH group should always be in excess. Typically, the catalyst ispresent in an amount ranging from 1 to 100 parts per million (ppm)platinum based on the total composition.

This polymerization reaction is described in the aforementioned patentswhich are hereby incorporated by reference to describe thepolymerization reaction used in the subject invention and to describe avariety of suitable materials. These materials would include thosematerials described in the above patents which have a suitable viscosityand which quickly react to form crosslinked siloxane polymers.

The components are individually preheated to a temperature in the rangeof from about 80° C. to about 200° C. and introduced into the subjectwire coater 10, under a pressure in the range of from about 200 psi toabout 3500 psi through inlet ports 12 and 14 (see FIG. 1).

It is to be emphasized that the viscosity of both components is lowenough to allow the applied pressure be the only significant forceacting on the coating material and that this pressure literally forcesthe resin through the coater 10. Therefore, the speed of the overallcoating or extrusion process is dependent primarily upon, andcontrollable by, the initial pressure on the components as they enterthe subject coater 10.

The rate of cure must be closely correlated with the resin throughputrate of the process since the resin must be cured to a substantiallyself-sustaining state as it leaves the die. If the resin cures tooquickly the coater 10 will be plugged, while, on the other hand, if theresin cures too slowly, the coating on the wire as it emerges from thewire coater 10 will not maintain a uniform thickness. The temperature ofthe reacting mixture of the "liquid polymer" components, the catalystconcentration and the concentration of the reacting functionalities arepreferably adjusted to provide a cure time of about one second. Then,the pressure under which the ingredients enter the wire coater can thenbe adjusted to provide a residence time of the reacting mixture in thewire coater 10 of from about 0.1 to about 0.4 of the curing time of themixture at the given pressure.

The above description relates to the production of a thermosetting(i.e., crosslinked polysiloxane wire coating). However, it is emphasizedthat this invention may also be used to apply a coating of otherthermosetting "liquid polymer" systems. However, the prepolymers,precursors, or ingredients of preferable candidate systems should haveviscosities, and cure times similar to those of the preferredpolysiloxane systems. Suitable examples would include the polyurethaneliquid polymers which have received so much interest from the injectionmolders and the polysulfide liquid polymer which has been developed andmarketed by the Thiokol Corporation.

The polyurethane systems typically involve the amine catalyzed reactionof an isocyanate (NCO) functionality with a hydroxyl (OH) functionality.The NCO containing compound is typically toluene diisocyanate and thehydroxyl containing compound is typically a polyoxyalkylene polyol. Thepolysulfide reaction typically involves the peroxide catalyzed reactionof an SH endcapped polyoxyalkylene with either another SH or a point ofethylenic unsaturation.

In evaluating a new candidate, important factors would include theviscosity of the individual components at a suitable reactiontemperature and the cure time once the components are mixed at thattemperature.

Referring to FIG. 1, the subject wire coater 10 comprises a barrel 26which is mounted on a supporting base member (not shown). A mandrel 20is rotatably mounted on said base member and disposed in the barrel 26.The mandrel has a tapered end 25 which tapers towards a die 16 which isattached to and held in place by the barrel 26. The wire 36 passesthrough a longitudinal axial channel in the mandrel 20 and is coated asit emerges from the mandrel 20. The heated liquid reactive componentsenter the wire coater 10, through inlet ports 12 and 14, pass throughdistribution rings 18 and 19, are blended in mixing chamber 23 and thenapplied to wire 36. The curing reaction reaches a point such that thecoating on the wire is in a substantially self-sustaining state as itemerges from die 16; the curing reaction then continues until acrosslinked elastomer is formed.

Preferably, the tubular product is extruded vertically upwards becausethe take-up equipment which receives the product from the extruder maybe adjusted to minimize the stress on the semi-cured extrudate as itemerges from the die 16.

The inlet ports 12 and 14 separately conduct the components to toroidaldistribution rings 18 and 19 which uniformly distribute the componentsaround the preferably rotating mandrel 20. The distribution rings areconcentric with the mandrel and are placed one behind the other.

As shown in FIGS. 1 and 2 each distribution ring receives a componentfrom an inlet port and then uniformly distributes the component aroundthe mandrel 20 and directs the flow of the component toward the mixingchamber 23. More specifically, and with reference to FIG. 2 theingredient flows through an inlet port into an annularly shapedreservoir 40 in the distribution ring 18. Exit ports 42 leading from theannularly shaped reservoir 40 are uniformly spaced around thecircumference of the mandrel 20. In FIG. 2 these exit ports 42 arecircular, however, their exact shape may be varied considerably as longas they provide a uniform distribution of the component around thecircumference of the mandrel 20.

Preferably, the total area of the exit ports 42 is less than the area ofthe inlet port 12. This will ensure that the inlet port 12 will have thecapacity to deliver the component to the annularly shaped reservoir 40,at a volumetric rate which is greater than the volumetric rate at whichthe exit ports can empty the reservoir 40. This is necessary to ensurethat the reservoir will always be full and thereby capable of uniformlydelivering the component to all points around the circumference of themandrel 20.

As also shown in FIG. 2 there is preferably a ridge 41 between each ofthe exit ports 42 on the inner surface 44 of the annularly shapedreservoir 40. These ridges ensure that there will be no "dead spots" inwhich small quantities of a component may be held for an extended periodof time. Such precautions should be taken in the design of allcomponents which form the flow path of the components and especially inthe design of the mixing chamber 23 and all points downstream from thatchamber, since once the reactants enter the chamber 23, they are mixedand the curing reaction beings. Any "dead spots" in the flow streamafter mixture will collect the reacting mixture and eventually plug thesubject wire coater 10.

In the design of the distribution rings 18 and 19 and the mixing chamber23, it is important that there be a minimum distance for the componentsto flow from the last ring 18 to the mixing chamber 23 (see FIG. 1).This is due to the reactive nature of the components and possibility ofthe two reactants coming in contact and initiating a premature reactionbefore they react the mixing chamber 23. It may be advisable to extendthe inner surface 21 of the distribution ring 18 to effectively separatethe two concentric streams of reactive ingredients.

Once the components leave the distribution rings 18 and 19, they willenter the preferably conical mixing chamber 23 defined by the inner wall24 of the barrel 26 and the surface 28 of the mandrel 20. As thecomponents pass through the mixing chamber 23, they are thoroughly mixedand begin to react to form the final cured product. The residence timeof the reacting mixture in the mixing chamber is carefully regulated toensure that the reaction does not proceed to the point that the resinsolidifies in the wire coater 10; this would require the disassembly andcleaning of the wire coater 10.

In accordance, with one preferred embodiment as shown in FIG. 1, themixing chamber 23 has a plurality of mixing fins 30 attaching to theinner wall 24 of the barrel 26 and second plurality of mixing fins 32attached to the mandrel 20. It is to be emphasized that the exactconfiguration of the internal structure of the mixing chamber 23 is notcritical to the practice of this invention so long as the mixing chamber23 is adequate to thoroughly mix the ingredients. In view of this, it isevident that the sets of fins 30 and 32 may be replaced with othermixing configurations such as threads, grooves or simply roughenedsurfaces.

Adequate mixing is a requirement of the subject process since if thereactants are not thoroughly blended the ultimate coating will not becontinuous; that is, there will be significant variations in the degreeof cure. This is not an acceptable condition. Preferably, the mandrel 20is rotated and the speed of rotation may be controlled to ensureadequate mixing.

In a particular embodiment, once the uniformly blended reacting mixtureleaves the mixing chamber 23, this mixture passes over the tip 34 ofmandrel 20 and onto the wire 36 which is moving at a speed of about 1000meters per minute (mpm) or more. The tip 34 provides a smooth surfaceover which the reacting mixture flows as it passes from the mixingchamber 23. In addition, the tip 34 positions the rapidly moving wire inthe center of the die opening 38. This is necessary to ensure that thecoating 40 has a uniform thickness.

The shape of tip 34 may be altered to transform the subject wire coater10 into an extruder for producing either tubing or solid rod. Forexample, if the wire were not used and a solid tip were substituted forthe hollow tip 34 shown in FIG. 1, the subject wire coater 10 wouldproduce a solid rod. Similarly, if the tip 34 were solid and equippedwith a projection which extended through the die, the subject wirecoater 10 would extrude a hollow tube.

It is noted that the subject device may be used to produce a rod, a tubeor a coated wire; however, the subject device will be most efficient andmost productive in producing a coated wire. The reason is that the wireprovides support for the curing extrudate as it emerges from the die.This support reduces the degree of cure of the extrudate necessary toachieve dimensional stability in the final cured product and thereforeallows higher production rates.

In designing the subject wire coater 10 for a particular application thefollowing calculations will be useful. Initially one must know thedimensions of the coated wire and the estimated speed at which the unitwill operate. For purposes of this example a wire having a 1.0 mmdiameter will be given a coating 1.5 mm thick. The design speed of theequipment will be 800 meters per minute (i.e., 13.3 meters/second). Itis assumed that the specific gravity of the coating material will beabout 1.25. Given these conditions the coating on the final product hasa linear density of about 15 grams per meter and the flow rate of thecoating material is about 195 grams per second.

In determing an appropriate volume for the mixing chamber, a reasonablecure time of the liquid polymer may be assumed to be about one second.The temperature of the ingredients as they enter the wire coater may beadjusted to provide the assumed cure time. Based on the above, thevolume of the mixing chamber should be in the range of from 15 to 45grams, since this would mean that a given volume of the material wouldhave a residence time in the mixing chamber of from about 10 percent to20 percent of its total cure time of one second. During the start-up ofthe wire coater it will be necessary to balance the temperature and thelinear speed of the wire to reach a range where the risk of plugging themixing chamber is reduced to a tolerable level and yet where the coatingresin is cured to a substantially self-sustaining state as it leaves thedie and is thereby able to maintain acceptable dimensional stability.With some specific coating materials it may be possible to increase theportion of the cure time which the material spends in the mixing chamberabove the aforementioned 20 percent level. This may be desirable, sincethe more time the ingredients spend in the mixing chamber the morethorough the mixing process.

Many aspects of the preferred embodiment which is described above may bemodified within the scope of this invention. For example, a threeingredient formulation may be employed by adding one additional inletport and distribution ring. It will also normally be necessary to add apump and a heater to supply the third ingredient at the propertemperature and pressure. However, if a third ingredient is added itwill be necessary to exercise care to ensure that no two mutuallyreactive ingredients are in contact for any significant period beforethey enter the mixing chamber 23. Other modifications will be readilyapparent to those skilled in the art in view of this specification.Therefore, the scope of this patent is not to be limited to the specificembodiments which have been described for illustrative purposes butrather by the following claims.

That which is claimed is:
 1. An improved method of rapidly coating awire with a crosslinked elastomer which is formed by mixing under heatand pressure multiple components of a liquid polymer system, said methodcomprising the steps of:(a) separately heating the components of theliquid polymer system to a temperature at which when the components aremixed the curing reaction will require from about 0.5 to about 3.0seconds, said components having a viscosity of from about 20 to about4000 centipoises; (b) separately pumping under pressure the heatedcomponents into an extruder barrel through separate inlet ports thereof,each port adapted to receive one of the heated components; (c) passing awire to be coated axially through a hollow rotatable mandrel locatedwithin said barrel and through a die attached to one end of said barrel,said mandrel having a tapered end tapering toward said die, said barreland the outer surface of the tapered end of said mandrel forming amixing chamber, (d) distributing the heated components around thecircumference of said mandrel in said barrel, (e) rotating said mandrelto cause mixing of the components in said mixing chamber, wherebypressure from said pumps causes extrusion of the mixed components ontosaid wire as it cures and passes through the die and coats said wire. 2.The method defined in claim 1 wherein distributing the heated componentsaround the circumference of said mandrel is accomplished by a pluralityof toroidal distribution rings disposed inside the barrel andsurrounding the mandrel, each ring receiving one component from one ofthe inlet ports.